This invention involves a structure for utilizing the known photocatalytic effect of certain metal oxides such as titanium dioxide. More particularly, it involves a combination of photocatalytic metal oxides on particular carrier surfaces of fiber glass.
The use of metal oxides such as titanium dioxide to photocatalytically decompose organic material has been extensively treated in the prior art. In addition to titanium dioxide, the photocatalytic effect has been reported to have been achieved with the oxides of zinc, tungsten, and tin. The present invention is expected to be useful with any photocatalyst that can be coated onto fiber glass. These may include some known photocatalytic non-oxide substances as well, but the ease of depositing the metallic oxides by presently known techniques makes them the preferred category for use with extended surface area substrates.
The utility of the photocatalytic compounds depends largely upon providing a solid carrier with large surface area so as to increase the contact area with the liquid or gas being treated. In U.S. Pat. No. 5,045,288 a layer of catalyst particles is loosely supported on a filter or in a granular bed. A more practical approach is to coat a solid support member with the catalyst. For example, use of porous ceramic substrates to support titanium dioxide coatings is disclosed in U.S. Pat. No. 5,035,784. Because the photocatalytic effect requires exposure of the catalyst to ultraviolet radiation, the use of a transparent substrate such as glass has been suggested. In particular, it has been recognized in U.S. Pat. Nos. 4,892,712; 4,966,759; and 5,032,241 (all to Robinson et al.) that fiberglass combines both transparency and high surface area, whereby carriers made from matrices of fiberglass are ideally suited for this purpose. The Robinson et al. patents mention both woven and non-woven fiber glass substrates, but woven meshes are preferred, and no details regarding non-woven embodiments are provided.
A non-woven fiber glass mat would be preferred over a woven mesh because substantially greater surface areas are attainable with a non-woven mat. Sizing compositions are typically applied onto fiber glass in order to reduce abrasion during processing. Additionally, when fiber glass is made into mats, it is common to apply organic binders to provide structural integrity to the mat structure. These sizings and binders, although useful to the manufacturer of the fiber glass mats, are a detriment to their use by makers of photocatalytic matrices. The organic polymer content of the sizings and binders significantly reduce adhesion of non-polymeric coatings such as the photocatalytic materials to the fiber glass mats. Attempts to use fiber glass mats as substrates for photocatalytic applications have entailed the added step of removing at least some of the organic content by heat cleaning (i.e., removing organics by combustion or volatilization at high temperature). Moreover, since it is desirable to maintain the structural integrity of the mat during processing at the user's facility, the heat cleaning step is generally carried out immediately prior to applying the photocatalytic material. Eliminating this step would be highly desirable to manufacturers of photocatalytic devices who wish to use fiber glass mat substrates.
It is also desirable for a photocatalyst support material to present a relatively low pressure drop to the flow of fluid (particularly air) therethrough. To achieve low pressure drop with an non-woven fiber glass mat would normally require maintaining the density of the mat low. But reducing the density also reduces the strength of a mat that is needed to be able to handle it in the normal processing operations involved with converting the mat to a photocatalytic matrix. Strength can be provided to a mat by organic binders, but adding such binders would be at odds with the objective of minimizing the organic content of the mat.